1. Technical Field
The present invention relates to fixing devices; in particular, it relates to fixing devices in image reproducing apparatus employing electrophotographic technology, such as photocopiers, laser printers and fax machines.
2. Description of Related Art
A conventional image reproducing apparatus capable of dual-sided printing will be explained using FIG. 1. Sheets are taken out one at a time from a feeder cassette 1 by a sheet-feeding roller 11. After a sheet passes through a transport path 21, its leading edge is aligned at the registration unit 22 and the sheet awaits further processing. The sheet is then transported to an image reproduction unit 4 at a predetermined timing. At the image reproduction unit 4, a toner image is formed on a photosensitive drum 41, and this toner image is transferred to a first surface of the sheet by a transfer apparatus 45. The sheet with an unfixed toner image transferred onto one surface passes through a transport path 24 and is transported to a nipping region between a heating roller 51 and a pressure roller 52, which constitute a fixing device 5. In this fixing device 5, the two rollers grip and heat the sheet. The unfixed toner image formed on the first side of the sheet is thereby fixed.
Next, the sheet having an image on the first side is sent to an upstream-end transport path 25 by means of a branching claw 6, and passes through the upstream-end transport path 25 to be passed to a switchback mechanism 7. Within the switchback mechanism 7, the transport direction of the sheet is changed and it is passed to a downstream transport path 26. Next the sheet having an image on the first side passes through the downstream transport path 26, a lower transport path 27, and a vertical transport path 28 and is again sent to the registration unit 22. There the front edge of the sheet is aligned, and the sheet awaits further processing. The sheet is then transported to the image reproduction unit 4 at a predetermined timing. At the image reproduction unit 4, a toner image formed on the photosensitive drum 41 is transferred to a second side of the sheet by the transfer apparatus 45. The sheet with an unfixed toner image transferred onto its second side is transported to the nipping region between the heating roller 51 and pressure roller 52, and the two rollers grip and heat the sheet. By this means, the unfixed toner image formed on the second side of the sheet is fixed. The sheet with images thus printed on both sides is sent to a paper discharge unit 29 by the branching claw 6, and is discharged to a paper discharge tray 9.
In an apparatus using the conventional technology as described above, the fixing process causes a sheet with an image formed on its first side to reach a high temperature. This sheet is immediately transported again to the image reproduction unit 4 while still hot in order to form an image on the second side. When the sheet makes contact with the photosensitive drum 41, heat from the sheet is transmitted to the photosensitive drum 41, and the temperature of the photosensitive drum 41 rises gradually. For example, if dual-sided printing is performed continuously for 100 sheets, the surface temperature of the photosensitive drum 41 rises to approximately 60.degree. C. If the temperature of the photosensitive drum 41 rises to approximately 60.degree. C., then the surface potential in the development regions of the photosensitive drum 41 decreases, and toner used by the image reproduction unit 4 begins to clump on the surface of the photosensitive drum 41, resulting in inferior images. Also, due to the influence of the heat of the photosensitive drum 41, the cleaning performance of the cleaning unit declines, or otherwise various out-of-order situations arise.
Table 1 sets forth sheet temperature and local temperatures in this sort of conventional image reproduction apparatus when continuous dual-sided printing is carried out.
Conditions for this Identification Test 1 are as follows.
Room temperature/humidity: 28.degree. C./50% PA1 Conveyance speed: 350 mm/sec, 40 sheets/min (A4 size , lateral through) PA1 Photo sensitive drum diameter: 60 mm PA1 Photosensitive drum heater temperature: 45.degree. C. (controlled to turn off at or above 45.degree. C.) PA1 Heating roller: One wherein upon printing 100 consecutive sheets without putting the heater on, an approximately 40.degree. C. heating roller temperature drop from 180.degree. C. is evident. PA1 Heating roller temperature: 180.+-.5.degree. C. (turned on at or below 175.degree. C., off at or above 185.degree. C.) PA1 How paper passed: Dual-sided printing every six sheets in turn; regular A4-size sheets, sideways sheet-passing. PA1 Room temperature/humidity: 28.degree. C./50% PA1 Transport speed: 350 mm/sec, 40 sheets/min (A4 size, lateral through) PA1 Photosensitive drum diameter: 60 mm PA1 Photosensitive drum heater temperature: 45.degree. C. (controlled to turn off at or above 45.degree. C.) PA1 Heating roller: One wherein upon printing 100 consecutive sheets without putting the heater on, an approximately 40.degree. C. heating roller temperature drop from 180.degree. C. is evident. PA1 Heating roller temperature: 160.+-.5.degree. C. (turned on at or below 155.degree. C., off at or above 175.degree. C.) PA1 How paper passed: Dual-sided printing every six sheets in turn; regular A4-size sheets, sideways sheet-passing.
(In the tables in the present specification, ".circle-w/dot." indicates "satisfactory," while ".DELTA." indicates "poor.")
TABLE 1 CONSECUTIVE DUAL-SIDED PRINTING TEST ACCORDING TO CONVENTIONAL TECHNOLOGY 1 Number of sheets Measurement Item 1 12 36 48 72 102 150 198 Sheet Temperature (.degree. C.) 60 60 60 60 60 60 60 60 Photosensitive Drum 45 47 50 54 55 58 60 58 Temperature (.degree. C.) Fixing Performance .circleincircle. .circleincircle. .circleincircle. .circleincircle. .circleincircle. .circleincircle. .circleincircle. .circleincircle. (Subjective Evaluation) Notes -- -- -- -- -- *1 *1 *1 *2 *1: image fogging occurred *2: offset image (cleaning defects)
As is clear from Table 1, upon continuous printing of two sides of approximately 100 sheets, the surface temperature of the photosensitive drum rises to approximately 60.degree. C., and thereafter remains near 60.degree. C. The fault noted in *1 is thought to be image fogging due to reduction of the surface potential in development regions.
The cause of the fault noted in *2 is thought to arise because the photosensitive drum temperature rises to approximately 60.degree. C., so that the surface toner temperature approaches the glass transition temperature, causing toner to adhere more readily to the photosensitive drum surface.
Fixing performance was evaluated by rubbing with the hand to determine whether the fixed toner image comes off or not. Here fixing posed no problems in particular.
In order to resolve the aforementioned problems, generally, dual-sided printing is performed at a temperature set lower than the temperature set for fixing during printing onto a single side only. The use of this kind of technology resolves the aforementioned problems in many image reproduction devices. Nonetheless, it has been realized that wherein conditions that were employed in the current experiments--such as no intermediate tray being provided (stack-less); temperature characteristics in the photosensitive drum being severe (the drop in surface potential due to temperature elevation is large); glass transition temperature of the toner being low; and printing sheet count being large--overlap, then out-of-order situations arise even according to the foregoing technology. Table 2 sets forth the sheet temperature and the temperatures at different locations during continuous dual-sided printing in an image reproducing device based on the conventional technology described above. The conditions for these confirmation experiments 2 are as follows.
TABLE 2 CONSECUTIVE DUAL-SIDED PRINTING TEST ACCORDING TO CONVENTIONAL TECHNOLOGY 2 Number of sheets Measurement Item 1 12 36 48 72 102 150 198 Sheet Temperature (.degree. C.) 60 60 60 60 60 60 60 60 Photosensitive Drum 45 47 50 50 49 50 52 52 Temperature (.degree. C.) Fixing Performance .DELTA. .DELTA. .circleincircle. .circleincircle. .circleincircle. .circleincircle. .circleincircle. .circleincircle. (Subjective Evaluation) Notes -- -- -- -- -- -- -- --
As is clear from Table 2, upon continuous printing of two sides of approximately 50 sheets, the surface temperature of the photosensitive drum rises to approximately 50.degree. C., and thereafter remains near 50.degree. C. The increase in temperature of the photosensitive drum is suppressed considerably, and the image fogging, cleaning failure and other faults occurring in the confirmation experiments 1 are resolved.
However, it was confirmed that fixing is poor on the initial several sheets. The cause is thought to be that with the photosensitive drum and transport paths of the image reproducing apparatus in a cooled state, the fixing temperature is set low, resulting in poor fixing.
It may be possible to resolve this second problem by, for example, setting the temperature setting slightly higher during dual-sided printing; but it would be dangerous to raise the temperature above the existing range.